What is the utility of OnDraw() function in CView class in Doc/View?

OnDraw

CView class is to represent the view of the document associated with it. A document can contain graphical drawing, text, and other objects. View should draw the appropiate output to the screen. OnDraw() event which should draw the state of the document in GUI. It is called whenever a document is refreshed i.e. on new and open event. MFC framework invalidates the window after a new document is created or a new document is opened by user. Windows GDI sends a PAINT event to main frame and MFC message handler calls OnDraw() event.

OnDraw() call stack

CMySDIView::OnDraw()
CView::OnPaint()
CWnd::OnWndMsg()
CWnd::WindowProc()
AfxCallWndProc()
AfxWndProc()
AfxWndProcBase()

OnDraw() Demo

In our application we have taken string a member named m_strFile for our CDocument. We are storing the path of the file as a state and printing the same in view.

class CMySDIDoc : public CDocument
{
protected: // create from serialization only
  CMySDIDoc();
  DECLARE_DYNCREATE(CMySDIDoc)

// Attributes
public:

// Operations
public:

// Overrides
public:
  virtual BOOL OnNewDocument();
  virtual void Serialize(CArchive& ar);

public:
  virtual ~CMySDIDoc();
  CString  m_strFile;
protected:
    
// Generated message map functions
protected:

  DECLARE_MESSAGE_MAP()
};

/* Loading "Untitled" in New document */
BOOL CMySDIDoc::OnNewDocument()
{
  if (!CDocument::OnNewDocument())
    return FALSE;

  m_strFile = "Untitled";
  return TRUE;
}

/* Open or Save Event */
void CMySDIDoc::Serialize(CArchive& ar)
{
  /* Saving the path to the file */
  if (ar.IsStoring())
  {
    m_strFile = ar.GetFile()->GetFilePath();
  }
  else /* Loading the path from file */
  {
    m_strFile = ar.GetFile()->GetFilePath();
  }
}

OnDraw() override

Now MFC doc-view framework calls this OnDraw() virtual function of CView after a new document is loaded or existing document is opened. We are drawing the m_strFile as a text in the view. So new document creates an output of "Untitled" text and open document shows the path of the file.

void CMySDIView::OnDraw(CDC* pDC)
{
  RECT rect;
  CMySDIDoc* pDoc = GetDocument();
  ASSERT_VALID(pDoc);
  pDC->GetWindow()->GetClientRect(&rect);
  pDC->DrawText(pDoc->m_strFile, &rect, 0);
}

OnDraw after OnNewDocument

OnDraw after OnOpenDocument

About our authors: Team EQA